I have read that Europa, one of Jupiter's moons
has water on it. Why is that and do the other
moons have water too? If not, why not?

Question Date: 2002-02-14

Answer 1:

Jupiter is 5 times farther from the Sun than we
are, so it and its moons only receive 1/(5*5) = 4%
of the sunlight that we do. Thus the surface of
Europa is very cold, and covered with ice several
miles thick. However, Europa is warmed from the
inside out by friction from gravitational tidal
forces. (These are the same forces by which the
Moon causes tides on Earth, but on Europa they
work a bit differently.)

The gravitational
pull of an object increases as one gets closer to
it, so on the Moon-facing side of Earth the Moon
pulls most strongly, and it hauls up a big mound
of water: a tide. In order for our tides to cause
friction, the difference in the Moon's pull on
different pieces of the Earth must be constantly
changing. The average force is not important- only
the differences (stretches) between separate parts
of an object are important, and the stretches must
constantly change in order to get friction and
heating. You can test this with a rubber ball:
Putting a large rock on top stretches it but does
not heat it up, but beating it repeatedly with a
mallet does heat it up. Only changing stretches
cause the ball to heat up.

In Earth's case,
the Moon's force differences change because of the
Earth's rotation. When our side of the Earth faces
the Moon, the Moon pulls our side more strongly
than the opposite side of the Earth. Half a day
later, the Moon pulls the opposite side more
strongly than it pulls our side. Ocean tides slosh
around the surface, following the Moon and Sun,
and causing slight frictional heating as they go.
Rocks also stretch and heat a little in response
to tidal forces, but liquids are most effective at
converting tidal forces into heat.

Europa
always keeps the same side towards Jupiter, so
rotation cannot produce the force difference
changes. (Europa used to rotate in the distant
past, but tidal forces gradually slowed it- will
skip that explanation). Europa gets its force
difference changes from its slightly non-circular
orbit around Jupiter: As it swings closer the
forces on each side increase, but the forces on
the side closest to Jupiter increase more than the
forces on the outward side. Jupiter's mass is
enormous (about 25,000 times more than our Moon),
so in
its great gravity field even a slight deviation
from a circular orbit can produce enough changing
stretch forces to gradually heat its moons from
the inside out.

The presence of three other
large moons constantly changing their positions
relative to Europa also adds tidal heating.
Europa gets enough tidal heating to melt the
deep ice and maintain an ocean which probably has
more water than Earth's. There might also be
hydrothermal vents and hot springs on the bottom
of Europa's ocean. In the absence of sunlight, any
life would have to subsist entirely on chemicals
cooked up by the hot springs. On Earth whole
ecosystems of strange creatures thrive near hot
springs in the deep ocean, without getting any
energy from the sun.

The next moon in from
Europa is Io, which gets even more gravitational
torture from being closer to Jupiter. There is so
much tidal heating that all water has boiled away-
no ocean. In fact, Io is the most volcanically
active body in the solar system. Volcanoes spew
sulfur in great fountains over 100 miles tall.

In the 20 years between visits from the
Voyager and Galileo probes, enough lava flowed to
noticeably change the map of Io.

The other
large moons, Ganymede and Callisto, seem to
have
oceans of water. Since they are farther from
Jupiter and experience less tidal heating, their
oceans are probably smaller and deeper than
Europa's. Europa has an icy surface with many
cracks, resembling the ice over the Arctic Ocean.
Ganymede and Callisto give no obvious surface
clues about their oceans, but the Galileo probe
can detect them indirectly. Saltwater conducts
electricity, and this causes a salty ocean to
produce a certain magnetic field in response to
Jupiter's changing magnetic field. Galileo can
detect the magnetic fields of the moons as it
flies close, and this is our only clue about
possible deep oceans inside these other moons.

Answer 2:

Well there is a fair amount of water in the solar
system. In fact, Hydrogen, the Hydrogen of
H2O (water)
is the MOST ABUNDANT element in the UNIVERSE. And
oxygen is not far behind (its the 4th most
abundant element). So the potential is, if
conditions are right for the Hydrogen and Oxygen
to come
together to form H2O (water).

Comets are
made up of water in the solid state (ice) and it
is estimated that there are trillions and
trillions of comets in the solar system. So water
is quite abundant as a solar system material!!!

All
of the Galilean satellites of Jupiter except Io
have a lot of water present as ice or perhaps as
liquid (water). Because these bodies are so
far from the sun, its too cold to have liquid
water AT THE SURFACE. However, deeper within the
moons (Europa, Callisto and Ganymede) it is hotter
because like on Earth, heat from the deep interior
left over from when the planets and moons formed
is escaping towards the surface. Hence at a depth
a few kilometers ( 1 mile = 1.6 kilometer) below
the surface, temperatures are high enough so that
water ice becomes water liquid!.

There are
below ground level large oceans of water on Europa
and probably also on Callisto and Ganymede !!!

Answer 3:

Since hydrogen and oxygen are very common in the
solar system, it is probably no great surprise
that water in its various forms is often found on
planets and their satellites. Many of the planets
and moons in the outer solar system have water ice
or water vapor.

What is exciting about
Europa is that there might be liquid water under
the ice. While water ice does not support life
very well, liquid water does (and I believe is
necessary for all the types of life we know about
on Earth), and if there is liquid water on Europa
then perhaps there is some form of life there as
well.

Finding life of any kind on another
planet or moon would be an incredible discovery.
Scientists at NASA and elsewhere are expending a
lot of effort onlooking for evidence of
extra-terrestrial life; it is a topic of great
interest to many people.